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hydrophobicity... how repellent is it?

Architect's memo 65: April 2001

Water, the odd chemical that really should be a gas but isn't, truly is the great mover and shaper of the world. It lowers mountains, scours out valleys, shapes coastlines, as well as being provider and/or host to every known life form on earth.

Wonderful stuff really, but it can also be a damned nuisance: in fact keeping water out of our buildings consumes a lot of our combined energies. All of us are familiar with methods used to achieve this:- flashings, free draining structures, sealants, and membranes to mention but a few. However even simple contact with water can be detrimental resulting in dirt and mould build-up along with heat loss.

Fully hydrophobic surfaces, that is surfaces that are simply not wetted by water, are very attractive in concept and can be achieved by the application, for example, of a substance such as wax. Wax however is subject to oxidation, and oxidation renders a hydrophobic surface hydrophilic (water loving).

Hydrophobic materials that resist oxidation are few and far between with the silicones being notable exceptions. Use of them both in polishes and masonry water repellents has been very successful. Modern masonry repellents based on siloxanes react with silica present in cement, sand, stone, etc and semi-permanently render the surface hydrophobic.

Incorporation of such materials into a standard paint does not confer the same sort of repellency as they do to mineral surfaces. The reason is two-fold, one is because the chemistry of paint resins does not easily react with siloxanes but the second is more interesting. Studies of naturally hydrophobic materials, such as the feathers on a duck, have shown that it is not only the oil that is important but the fact that the feather has a fine profiled surface.

It appears that a micro-textured hydrophobic surface offers less points of contact to the water droplet so increasing the water repellent 'ducks back' effect. This offers the possibility of producing long-term durable water-repellent paints. The theory is to produce a highly filled paint using relatively coarse silaceous extenders to produce a porous, slightly rough surface. Use of silicone resins and reactive siloxanes can render the surface of the pores hydrophobic which, coupled with a micro-rough surface, can result in long-term water repellency.

Such paints with an attractive flat surface and very high breathability may achieve a useful niche in the scheme of things.

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